It’s the most dangerous food poisoning bug you’d probably hardly ever heard of – at least until a few days ago.

But now listeria is making headline news and it’s not pretty.

The bacterium has caused nine deaths across Europe and recently,  43 frozen mixed vegetable and sweetcorn products – from Lidl, Iceland, Sainsbury’s, Tesco and Waitrose – are currently being recalled because they might contain the potentially deadly bug.

If you’ve got any of these veg in your freezer, advice from the Food Standards Agency is to throw them out straight away. If you haven’t, all well and good, but it’s still worth learning how to keep frozen food safe.

“Things can go wrong with the way you handle frozen food before it goes in the freezer and when it comes out, but also while it’s in there,” says Dr Lisa Ackerley, aka the Hygiene Doctor, who has over 35 years’ experience in environmental health.

Here are the top tips to stop your freezer from making you ill…

1. Cook veg if the pack says so

Many supermarket packs of frozen vegetables must be cooked, so check the label. In the case of the current listeria outbreak, illness has probably resulted from eating thawed sweetcorn in salads and sandwiches. If it had been cooked, the bug would have been killed.

“It‘s very important to check the label and be aware that if the food is not ready-to-eat then it could be dangerous without cooking,” says Dr Ackerley.

You may be surprised to find that some fruits – berries, for example – should be cooked so don’t use these to top your morning porridge straight from the freezer.

2. Make sure food is safe when it goes in

Harmful bacteria will rarely grow at freezer temperatures but you can accidentally make food unsafe before you put it in.

If you are freezing food you’ve cooked yourself be careful about how you cool it down.

Dr Ackerley says that if food hangs around too long at a temperature that isn’t cold enough (below 5C) to suspend the activity of any bacteria that might be present, but isn’t hot enough (above 70C) to kill them either, numbers can rise to huge levels.

If you then put food like this in the freezer, the bacteria won’t be killed – they’ll just go into a state of suspended animation ready to grow some more and possibly make you ill
if you don’t nuke it enough when it comes out.

The best idea is to get food down to fridge temperature before you put it in the freezer and to split large batches of hot food into smaller ones to allow quicker cooling.

3. Thaw it right

It takes longer, but thawing out frozen food, especially bigger items like joints of meat is safer done in the fridge – taking care to not let thawing raw meat contaminate any ready-to-eat food in your fridge.

On the counter, at room temperature, you can get parts of food that get warm enough to allow bacteria to grow out of control.

Another method that works, especially for whole chickens and turkeys is to immerse them in water, adding ice cubes to keep the water chilled. When the ice has all melted it’s your clue that the water is getting too warm. At this point, add some more until the poultry has thawed all the way through.

It’s possible to thaw smaller portions of foods in the microwave too, but if you do so, always go on to cook the food straight after it has thawed.

4. Stash it properly

If your ice cubes are jostling for space with your fish fingers and frozen meat, your freezer-packing skills need an overhaul.

“Just as you would in your fridge, meat, fish and poultry should be kept in a separate area of the freezer – ideally the bottom shelf – so no contamination from the packaging can get on to any ready-to-eat food,” says Dr Ackerley.

5. Wrap it up tight

Tightly wrapping up your frozen food in freezer bags will go a long way to keeping it safe.

This includes having clips on bags of vegetables and fruit that could otherwise spread all over the freezer and – if you’re unlucky – spread listeria too.

Being airtight is important to keep food tasting good too, as if air gets to the surface of your food – meat especially – it can cause freezer burn, which is when it dries out and can develop rancid flavours.

“Use proper freezer bags rather than clingfilm,” says Dr Ackerley. “They are more robust and less likely to break apart.”